The present invention relates to a putter head.
A putter head has been proposed in which a face of a head body is fitted with a face insert made of a material, that is different from that of the head body. For example, Japanese Patent Application Publication No. 2007-117472 describes a putter head in which a face of a head body is fitted with a face insert which has a two-layer structure made up of a high-hardness layer and low-hardness layer. As examples of face insert materials, the patent document cites synthetic resins such as ionomer resins and polyurethane resins, thermoplastic elastomers such as polyurethane elastomers and polyester elastomers, and rubbers such as styrene-butadiene rubber and butadiene rubber.
Also, Japanese Patent Application Publication No. 2007-117634 describes a putter head fitted with a face insert which has a two-layer structure made up of a high-hardness layer and a low-hardness layer, wherein in a central area of the face insert, a projection is formed on the high-hardness layer and a depression is formed in the low-hardness layer such that the projection will be buried in the depression.
Normally, putter heads are fitted with a face insert made of a polymer in order to provide a soft feel of hitting a ball. Normally, the lower the hardness of the polymer used, the softer the hitting feel. However, the roll of the ball, which is related to the smash factor, varies with the polymer used. For example, in the case of thermoplastic polyurethane elastomers, since higher hardness normally tends to result in a higher smash factor, the use of a low-hardness thermoplastic polyurethane elastomer to obtain a soft hitting feel will result in a low smash factor. Consequently, it is difficult to provide an appropriate putter for a golfer who prefers a soft hitting feel, but wants a putter with a high smash factor. Conversely, it is also difficult to provide an appropriate putter to a golfer who prefers a hard hitting feel, but wants a putter with a low smash factor.
On the other hand, putter heads generally have a loft angle of approximately 2 to 5 degrees, and it has been found by means of high-speed photography that backspin is imparted to a golf ball upon impact. The backspin causes the ball to slide over a green for a while, and then changes to overspin, causing the ball to roll over the green. A large amount of backspin decreases stability of a putt. A putter head which reduces backspin is described in Japanese Patent Application Publication No. 2008-154974. The putter head has plural grooves formed substantially in parallel in its face surface. A ratio W/S between groove width W (mm) and groove-to-groove spacing S (mm) is 0.5 to 2.0.
The parallel grooves formed in the face surface reduce the amount of backspin of the putted ball, causing the ball rolling over the green to go into overspin at an early stage after the putt. Consequently the ball rolls smoothly along the line aimed at. However, the formation of plural grooves in the surface of the face greatly reduces the contact area of the surface with the ball, changing the hitting feel and reducing initial ball speed and thus the smash factor. On the other hand, a high smash factor increases the initial ball speed, making the ball liable to slide over the green. The sliding of the ball over the green dampens the force of the ball, often causing a putt to come up short.